Increasing impacts of extreme winter warming events on permafrost

Winter warming events (WWE) are abrupt and short-lasting (hours-to-days) events of extraordinarily warm weather occurring during wintertime, sometimes accompanied by rainfall (rain on snow events; ROS). Through direct heat transfer and changes in the snowpack properties, these events cause changes in the below-ground thermal regime that, in turn, controls a suite of ecosystem processes ranging from microbial activity to permafrost and vegetation dynamics. In addition, the overall impacts of WWE on ground temperatures may also depend on the timing of the events and the preceding snowpack characteristics. The frequency and intensity of these events in the Arctic, including the Swedish subarctic, has increased remarkably during the recent decades and is expected to increase even further during the 21st Century. In addition, snow depth (not necessarily snow duration) is projected to increase in many parts of the Arctic. In 2005, a manipulation experiment was set up on a lowland permafrost mire in the Swedish subarctic, to simulate projected future increases in winter precipitation. In this study, we analyse this 15-year record of ground temperature, AL thickness, and meteorological variables, to evaluate the influence of different types and timings of WWE on permafrost under different snowpack conditions. We found that all WWE types were strongly linked to permafrost warming in both the ambient and the manipulated plots during the winter months, but that significantly warmer summer ground temperatures and, consequently, thicker active layer (AL) were only linked to ROS events in manipulated plots. Additional long-term AL thickness data from nearby permafrost sites indicate an increasingly stronger influence of WWE over time since the mid-1990s, to the detriment of the influence of summer air temperatures.